This invention relates to molten metal such as molten aluminum, and more particularly, it relates to an improved method for fluxing molten metals such as molten aluminum using improved shaft design.
The use of a chlorine containing reactive fluxing gas, for the purpose of removing alkali elements (i.e., Na, Ca, K, Li), is a well established practice in the treatment of molten aluminum. Under equilibrium conditions, the respective chlorides of these elements are produced as reaction products. With the exception of LiCl, all of these halide salts, as pure species, are solid at normal treatment temperatures and thus are easily separated to the melt surface as a supemate and are removed by skimming.
Alkali elements are usually present at melt concentrations less than 500 ppm. According to the law of mass action (reaction rate approximately proportional to the concentration of reacting species), non-equilibrium metastable salts such as AlCl3 and MgCl2 (if Mg is present) are generated. These halides are undesirable because they contribute significantly to process airborne emissions. Further, MgCl2 melts at 1306xc2x0 F. and is typically molten at normal melt treatment temperatures. Molten salts are highly undesirable because of the difficulty of removing to the surface for skimming. Thus, it is highly desirable to react or complex the alkali elements to produce higher melting salts which in solid form are more efficiently separated by flotation to the surface.
In the prior methods of dispersing fluxing gas, for example, in a molten aluminum body, the fluxing gas is introduced down a shaft into the body and dispersed by a rotating impeller mounted on the shaft. However, this method is not without limitations. The rotating impeller creates a vortex about the shaft that indicates that a large portion of the molten metal is swirling or circulating about the impeller shaft at a rate approaching the rotation speed of the impeller. Fluxing media added to the molten metal tends to circulate with the molten metal with only minimal dispersion. Further, the vortex has the effect of increasing the surface area of the molten body exposed to air. The increased exposure of the molten metal to air results in an increase in dross formation, subsequent entrainment of the dross and its detrimental collateral effects. When the fluxing material is a gas, the vortex creates a problem in yet another way. Fluxing gas is displaced towards the center of the vortex by body force separation with the result that other parts of the molten body are not adequately treated with fluxing gas. Thus, the effectiveness of the process is reduced because portions of the molten body do not get treated with fluxing material. In addition, fluxing gas entrained in the molten metal flow pattern tends to coalesce, resulting in larger bubbles of fluxing gas developing in the melt. The larger bubbles lower the effectiveness of the fluxing process because less molten metal gets treated.
Common methods employed to suppress vortex formation include the insertion of baffles or rods into the melt. However, baffles are undesirable because a dead volume develops behind the trailing edges of the baffle. Another method used to suppress vortex formation is to limit power input to the impeller. However, this severely limits efficiency.
These problems continue to plague the industry as indicated in U.S. Pat. No. 5,160,693, for example, which discloses that with rotating impellers a surface vortex forms, the vortex rotating about and flowing downwardly along the impeller shaft, thereby agitating surface dross and drawing impurities back into the melt. The patent also indicates that an ideal system would minimize disturbances to the surface dross to prevent recontamination of the treated melt.
Thus, there is a great need for a more effective fluxing process which suppresses ingestion of dross from the surface back into the melt by vortex formation, for example, maintains the fluxing material finely dispersed throughout the molten body, and intensifies the contact of molten metal with fluxing material for improved fluxing of the melt.
An object of this invention is to provide an improved treatment process for dispersing media in molten metal.
Another object of this invention is to provide an improved fluxing process for molten aluminum.
Yet another object of this invention is to provide an improved shaft for use in molten aluminum.
Yet a further object of the invention is to provide an improved fluxing process for molten aluminum using a rotating impeller wherein substantially no vortex is formed.
And yet a further object of the invention is to provide an improved process for a body of molten aluminum wherein the fluxing gas is finely dispersed throughout the body for improved contact of fluxing gas with metal.
Still, yet another object of the invention is to provide a process for providing increased shear forces in a body of molten metal for improved dispersion of treatment media, such as fluxing gases and salts, throughout the body.
And still a further object of this invention is to provide a process for fluxing molten aluminum wherein large amounts of fluxing gas can be added without entrainment or fuming above the melt.
These and other objects will become apparent from a reading of the specification and claims and an inspection of the accompanying drawings appended hereto.
In accordance with these objects there is provided a method of treating a body of molten metal passing through a treatment bay having a surface. The method comprises providing a body of molten metal in a treatment bay and projecting an impeller on a shaft extending into the treatment bay through the surface to contact the molten metal. The shaft is comprised of a carbon or graphite material. A wire mesh or fibrous reinforced refractory sleeve substantially inert to molten aluminum is fixedly positioned around the shaft extending above and below the surface of the molten aluminum, thereby greatly extending the useful life of the shaft.